The present disclosure relates to nitride semiconductor devices, and more particularly to nitride semiconductor devices which can be used as power devices in power supply circuits etc.
Nitride semiconductors represented by gallium nitride (GaN) are wide gap semiconductors. For example, GaN has a bandgap of 3.4 eV at room temperature. Aluminum nitride (AlN) has a bandgap of 6.2 eV at room temperature. Also, nitride semiconductors have high breakdown field strength and high saturated drift velocity of electrons, as compared to compound semiconductors such as gallium arsenide (GaAs) etc., or silicon (Si) semiconductors, etc. In a heterostructure (AlGaN/GaN heterostructure) of aluminum gallium nitride (AlGaN) and GaN, charges are generated at a heterointerface due to spontaneous polarization and piezoelectric polarization. A sheet carrier concentration of 1×1013 cm−2 or more is obtained even when AlGaN and GaN are undoped. A diode and a hetero-junction field effect transistor (HFET) having high current density can be provided by utilizing two-dimensional electron gas (2DEG) generated at the heterointerface. Therefore, power devices etc. made of nitride semiconductor which is advantageous in increasing output power and a breakdown voltage are now being actively researched and developed.
A Schottky diode is one type of diode used as a power device. A Schottky diode having an AlGaN/GaN heterostructure operates with a large current and low resistance, since two-dimensional electron gas generated at the interface between an undoped AlGaN layer and an undoped GaN layer is used as a channel. In general, a Schottky diode has excellent switching characteristics and a low forward threshold voltage as advantages. A disadvantage is, however, that a reverse leakage current is large.
In order to reduce a reverse leakage current of a Schottky diode, a method, in which two types of metal are used for an anode electrode, and the metal with a higher Schottky barrier is formed to cover the metal with a lower Schottky barrier, is suggested. (See, for example, Japanese Patent Publication No. 2005-317843). When a forward-direction voltage is applied to the diode, a current flows to the metal with the lower Schottky barrier, and a low threshold voltage can be thus maintained. On the other hand, when a reverse-direction voltage is applied to turn the diode off, the metal with the higher Schottky barrier reduces the reverse leakage current.
A passivation film is usually formed on a surface of a device as a surface protection film. The formation of the passivation film provides the advantages of reducing formation of a surface state and reducing the phenomenon called “current collapse” of reduction in a forward current. Since it has also the function of protecting the device from impurities, the passivation film needs to be formed in view of improving reliability of the device.